176 BALANID.E. 



** 



of symmetrical longitudinal ribs on their inner surfaces, 

 and the peculiar character of the narrow radii, hardly dif- 

 fering in structure from the parietes, are characters which 

 are rare in the Balaninae, but universal in the Chthamalinrc. 

 Hence, taking the whole organisation of the two species of 

 Pachylasma, it is certain that they must be ranked amongst 

 the Chthamalinse, though the leading character of the 

 group, namely, the rostrum being furnished, like the carina, 

 with ala3, here fails. Owing to this conviction, I examined 

 very young individuals of P. giganteum, and in specimens 

 only the iJgth of an inch in height, I was interested by 

 finding eight separate compartments, with the rostrum 

 having distinct alse ; hence, at this early age, as far as the 

 shell is concerned, this species may be said to be an Octo- 

 meris ; and we have seen that this likewise holds good 

 with the included animal's body; as the young shell in- 

 creases in size the minute rostrum and rostro-lateral com- 

 partments blend together (PI. 19, fig. bd), without even 

 traces of sutures being left. 



With respect to Pachylasma aurantiacum, I have no doubt 

 that at an early age it would possess a perfectly distinct 

 rostrum with alae; for in the one specimen which I have seen, 

 the compound rostrum is divided by sutures, faintly visible, 

 both externally and internally (PL 20, fig. \a,\ b), into three 

 compartments (b, a, b), of which the middle one, or true 

 rostrum, still shows, in the manner in which it underlaps the 

 little rostro-lateral compartments, vestiges of alae. I may re- 

 mark, that we have here the same structure as in Chelonobia, 

 formerly described, with the following differences, that here 

 the sutures pass through the outer lamina of the parietes, so 

 that, as seen externally, the separation of the three com- 

 partments is much more perfect than in Chelonobia ; on the 

 other hand, internally, the separation is less distinct, as the 

 two rudimentary rostro-lateral compartments do not form 

 part of the sheath. Pachylasma aurantiacum is further re- 

 markable, from the two lateral compartments (c, d) on each 

 side, tending to blend together, being only separated by 

 sutures not more distinct, externally, than those separating 

 the compound rostrum, but more distinct internally, for 

 they run up the sheath. From these facts it follows, that 



